I've tried using those pitch rate calculators but so much of it is guesswork that it just seems pointless. I have no idea how many billion cells (slurry density) I have in my harvested yeast.
When I harvest my yeast, I pour it into three 16oz mason jars. I like to leave about an inch of beer at the top as a vapor barrier when storing in the fridge.
When I make beer, a few days prior I'll pull out a jar of slurry and give it a whiff. If it smells good, I'll make a starter and let it whirl on the stir plate a couple days. If the starter smells ok, I'll decant what I can and pitch the rest in my wort. I can usually see fermenter activity within a couple hours, depending on the temp.
Sometimes tools come along which add more drudgery and very little benefit. Whether the pitch tool is one of these, I don't know. For me though, I like to keep the rules simple and see what I can get away with. I want to have fun and make enjoyable beer.